Against all odds society can rely on the deeds of good Samaritans. But in the biblical scenario there is no doubt the robbers' victim is in need of help. This dramatically limits the pool of potential helpers to passers-by. In a series of experiments, we try to understand to what degree, and in which ways, generosity is affected by uncertainty of the need for help.

In a first experiment, we manipulate the degree and type of uncertainty about the relative wealth of an anonymous recipient. Surprisingly, experimental dictators give more. We find that helpers are chiefly concerned about the risk of recipients being completely left without support.

In a second experiment, we find that potential donors are willing to spend extra money on making sure that their donation is not subverted (ie, goes into the hands of local warlords).

In a third experiment, we find that potential donors are more likely to make a donation if this risk is perfectly contained (in legal terms: if the contract grants specific performance), rather than merely monetary compensation.

Christoph Engel is the Executive Director of the Max Planck Institute for Research on Collective Goods in Bonn and full professor of law at the Universities of Bonn and Rotterdam.

Engel is Dr.jur and Dr.h.c. phil (Hebrew University Jerusalem). His main area of research is behavioural law and economics. Most of his papers are experimental. He presents two important research questions: in which ways does a behavioural perspective qualify the definition of the policy problems law is meant to address, and how can legal intervention solve or at least mitigate these problems by impacting the behaviour of its addressees? His papers have been published in peer reviewed legal journals, such as the Journal of Empirical Legal Studies, the Journal of Legal Studies and the Journal of Law and Economics; in economics journals, such as Experimental Economics or Economics Letters; and in psychology journals, such as Perspectives on Psychological Science and the Journal of Behavioral Decision Making.